European Space Agency
The European Space Agency is an intergovernmental organisation of 22 member states dedicated to the exploration of space. Established in 1975 and headquartered in Paris, France, ESA has a worldwide staff of about 2,200 in 2018 and an annual budget of about €5.72 billion in 2019. ESA's space flight programme includes human spaceflight; the main European launch vehicle Ariane 5 is operated through Arianespace with ESA sharing in the costs of launching and further developing this launch vehicle. The agency is working with NASA to manufacture the Orion Spacecraft service module, that will fly on the Space Launch System; the agency's facilities are distributed among the following centres: ESA science missions are based at ESTEC in Noordwijk, Netherlands. After World War II, many European scientists left Western Europe in order to work with the United States. Although the 1950s boom made it possible for Western European countries to invest in research and in space-related activities, Western European scientists realised national projects would not be able to compete with the two main superpowers.
In 1958, only months after the Sputnik shock, Edoardo Amaldi and Pierre Auger, two prominent members of the Western European scientific community, met to discuss the foundation of a common Western European space agency. The meeting was attended by scientific representatives from eight countries, including Harrie Massey; the Western European nations decided to have two agencies: one concerned with developing a launch system, ELDO, the other the precursor of the European Space Agency, ESRO. The latter was established on 20 March 1964 by an agreement signed on 14 June 1962. From 1968 to 1972, ESRO launched seven research satellites. ESA in its current form was founded with the ESA Convention in 1975, when ESRO was merged with ELDO. ESA had ten founding member states: Belgium, France, West Germany, the Netherlands, Sweden and the United Kingdom; these signed the ESA Convention in 1975 and deposited the instruments of ratification by 1980, when the convention came into force. During this interval the agency functioned in a de facto fashion.
ESA launched its first major scientific mission in 1975, Cos-B, a space probe monitoring gamma-ray emissions in the universe, first worked on by ESRO. The ESA collaborated with NASA on the International Ultraviolet Explorer, the world's first high-orbit telescope, launched in 1978 and operated for 18 years. A number of successful Earth-orbit projects followed, in 1986 ESA began Giotto, its first deep-space mission, to study the comets Halley and Grigg–Skjellerup. Hipparcos, a star-mapping mission, was launched in 1989 and in the 1990s SOHO, Ulysses and the Hubble Space Telescope were all jointly carried out with NASA. Scientific missions in cooperation with NASA include the Cassini–Huygens space probe, to which ESA contributed by building the Titan landing module Huygens; as the successor of ELDO, ESA has constructed rockets for scientific and commercial payloads. Ariane 1, launched in 1979, carried commercial payloads into orbit from 1984 onward; the next two versions of the Ariane rocket were intermediate stages in the development of a more advanced launch system, the Ariane 4, which operated between 1988 and 2003 and established ESA as the world leader in commercial space launches in the 1990s.
Although the succeeding Ariane 5 experienced a failure on its first flight, it has since established itself within the competitive commercial space launch market with 82 successful launches until 2018. The successor launch vehicle of Ariane 5, the Ariane 6, is under development and is envisioned to enter service in the 2020s; the beginning of the new millennium saw ESA become, along with agencies like NASA, JAXA, ISRO, CSA and Roscosmos, one of the major participants in scientific space research. Although ESA had relied on co-operation with NASA in previous decades the 1990s, changed circumstances led to decisions to rely more on itself and on co-operation with Russia. A 2011 press issue thus stated: Russia is ESA's first partner in its efforts to ensure long-term access to space. There is a framework agreement between ESA and the government of the Russian Federation on cooperation and partnership in the exploration and use of outer space for peaceful purposes, cooperation is underway in two different areas of launcher activity that will bring benefits to both partners.
Notable outcomes are ESA's include SMART-1, a probe testing cutting-edge new space propulsion technology, the Mars Express and Venus Express missions, as well as the development of the Ariane 5 rocket and its role in the ISS partnership. ESA maintain
A geodetic datum or geodetic system is a coordinate system, a set of reference points, used to locate places on the Earth. An approximate definition of sea level is the datum WGS 84, an ellipsoid, whereas a more accurate definition is Earth Gravitational Model 2008, using at least 2,159 spherical harmonics. Other datums are defined at other times. Mars has no oceans and so no sea level, but at least two martian datums have been used to locate places there. Datums are used in geodesy and surveying by cartographers and satellite navigation systems to translate positions indicated on maps to their real position on Earth; each starts with an ellipsoid, defines latitude and altitude coordinates. One or more locations on the Earth's surface are chosen as anchor "base-points"; the difference in co-ordinates between datums is referred to as datum shift. The datum shift between two particular datums can vary from one place to another within one country or region, can be anything from zero to hundreds of meters.
The North Pole, South Pole and Equator will be in different positions on different datums, so True North will be different. Different datums use different interpolations for the precise size of the Earth; because the Earth is an imperfect ellipsoid, localised datums can give a more accurate representation of the area of coverage than WGS 84. OSGB36, for example, is a better approximation to the geoid covering the British Isles than the global WGS 84 ellipsoid. However, as the benefits of a global system outweigh the greater accuracy, the global WGS 84 datum is becoming adopted. Horizontal datums are used for describing a point on the Earth's surface, in latitude and longitude or another coordinate system. Vertical datums measure depths. In surveying and geodesy, a datum is a reference system or an approximation of the Earth's surface against which positional measurements are made for computing locations. Horizontal datums are used for describing a point on the Earth's surface, in latitude and longitude or another coordinate system.
Vertical datums are used to underwater depths. The horizontal datum is the model used to measure positions on the Earth. A specific point on the Earth can have different coordinates, depending on the datum used to make the measurement. There are hundreds of local horizontal datums around the world referenced to some convenient local reference point. Contemporary datums, based on accurate measurements of the shape of the Earth, are intended to cover larger areas; the WGS 84 datum, identical to the NAD83 datum used in North America and the ETRS89 datum used in Europe, is a common standard datum. For example, in Sydney there is a 200 metres difference between GPS coordinates configured in GDA and AGD, an unacceptably large error for some applications, such as surveying or site location for scuba diving. A vertical datum is a reference surface for vertical positions, such as the elevations of Earth features including terrain, water level, man-made structures. In geodetic coordinates, the Earth's surface is approximated by an ellipsoid, locations near the surface are described in terms of latitude and height.
Geodetic latitude, resp. altitude, is different from geocentric latitude, resp. altitude. Geodetic latitude is determined by the angle between the equatorial plane and normal to the ellipsoid, whereas geocentric latitude is determined by the angle between the equatorial plane and line joining the point to the centre of the ellipsoid. Unless otherwise specified, latitude is geodetic latitude; the ellipsoid is parameterised by the semi-major axis a and the flattening f. From a and f it is possible to derive the semi-minor axis b, first eccentricity e and second eccentricity e ′ of the ellipsoid The two main reference ellipsoids used worldwide are the GRS80 and the WGS84. A more comprehensive list of geodetic systems can be found here; the Global Positioning System uses the World Geodetic System 1984 to determine the location of a point near the surface of the Earth. Datum conversion is the process of converting the coordinates of a point from one datum system to another. Datum conversion may be accompanied by a change of grid projection.
A geodetic reference datum is a known and constant surface, used to describe the location of unknown points on the Earth. Since reference datums can have different radii and different center points, a specific point on the Earth can have different coordinates depending on the datum used to make the measurement. There are hundreds of locally developed reference datums around the world referenced to some convenient local reference point. Contemporary datums, based on accurate measurements of the shape of the Earth, are intended to cover larger areas; the mos
Galileo (satellite navigation)
Galileo is the global navigation satellite system that went live in 2016, created by the European Union through the European GNSS Agency, headquartered in Prague in the Czech Republic, with two ground operations centres, Oberpfaffenhofen near Munich in Germany and Fucino in Italy. The €10 billion project is named after the Italian astronomer Galileo Galilei. One of the aims of Galileo is to provide an independent high-precision positioning system so European nations do not have to rely on the U. S. GPS, or the Russian GLONASS systems, which could be disabled or degraded by their operators at any time; the use of basic Galileo services will be open to everyone. The higher-precision capabilities will be available for paying commercial users. Galileo is intended to provide horizontal and vertical position measurements within 1-metre precision, better positioning services at higher latitudes than other positioning systems. Galileo is to provide a new global search and rescue function as part of the MEOSAR system.
The first Galileo test satellite, the GIOVE-A, was launched 28 December 2005, while the first satellite to be part of the operational system was launched on 21 October 2011. As of July 2018, 26 of the planned 30 active satellites are in orbit. Galileo started offering Early Operational Capability on 15 December 2016, providing initial services with a weak signal, is expected to reach Full Operational Capability in 2019; the complete 30-satellite Galileo system is expected by 2020. It is expected that the next generation of satellites will begin to become operational by 2025 to replace older equipment. Older systems can be used for backup capabilities. There are 22 satellites in usable condition, 2 satellites are in "testing" and 2 more are marked as not available. In 1999, the different concepts of the three main contributors of ESA for Galileo were compared and reduced to one by a joint team of engineers from all three countries; the first stage of the Galileo programme was agreed upon on 26 May 2003 by the European Union and the European Space Agency.
The system is intended for civilian use, unlike the more military-oriented systems of the United States and China. The European system will only be subject to shutdown for military purposes in extreme circumstances, it will be available at its full precision to both military users. The countries that contribute most to the Galileo Project are Italy; the European Commission had some difficulty funding the project's next stage, after several "per annum" sales projection graphs for the project were exposed in November 2001 as "cumulative" projections which for each year projected included all previous years of sales. The attention, brought to this multibillion-euro growing error in sales forecasts resulted in a general awareness in the Commission and elsewhere that it was unlikely that the program would yield the return on investment, suggested to investors and decision-makers. On 17 January 2002, a spokesman for the project stated that, as a result of US pressure and economic difficulties, "Galileo is dead."A few months however, the situation changed dramatically.
European Union member states decided it was important to have a satellite-based positioning and timing infrastructure that the US could not turn off in times of political conflict. The European Union and the European Space Agency agreed in March 2002 to fund the project, pending a review in 2003; the starting cost for the period ending in 2005 is estimated at €1.1 billion. The required satellites were to be launched between 2011 and 2014, with the system up and running and under civilian control from 2019; the final cost is estimated at €3 billion, including the infrastructure on Earth, constructed in 2006 and 2007. The plan was for private companies and investors to invest at least two-thirds of the cost of implementation, with the EU and ESA dividing the remaining cost; the base Open Service is to be available without charge to anyone with a Galileo-compatible receiver, with an encrypted higher-bandwidth improved-precision Commercial Service available at a cost. By early 2011 costs for the project had run 50% over initial estimates.
Galileo is intended to be an EU civilian GNSS. GPS reserved the highest quality signal for military use, the signal available for civilian use was intentionally degraded; this changed with President Bill Clinton signing a policy directive in 1996 to turn off Selective Availability. Since May 2000 the same precision signal has been provided to the military. Since Galileo was designed to provide the highest possible precision to anyone, the US was concerned that an enemy could use Galileo signals in military strikes against the US and its allies; the frequency chosen for Galileo would have made it impossible for the US to block the Galileo signals without interfering with its own GPS signals. The US did not want to lose their GNSS capability with GPS while denying enemies the use of GNSS; some US officials became concerned when Chinese interest in Galileo was reported. An anonymous EU official claimed that the US officials implied that they might consider shooting down Galileo satellites in the event of a major conflict in which Galileo was used in attacks against American forces.
The EU's stance is that Galileo is a neutra
ECEF known as ECR, is a geographic and Cartesian coordinate system and is sometimes known as a "conventional terrestrial" system. It represents positions as X, Y, Z coordinates; the point is defined as the center of mass of Earth, hence the term geocentric coordinates. The distance from a given point of interest to the center of Earth is called the geocentric radius or geocentric distance, its axes are aligned with the international reference pole and international reference meridian that are fixed with respect to Earth's surface, hence the descriptor earth-fixed. This term can cause confusion, since Earth does not rotate about the z-axis, is therefore alternatively called ECR; the z-axis extends through true north, which does not coincide with the instantaneous Earth rotational axis. The slight "wobbling" of the rotational axis is known as polar motion; the x-axis intersects the sphere of the earth at 0 ° longitude. This means that ECEF rotates with the earth, therefore coordinates of a point fixed on the surface of the earth do not change.
Conversion from a WGS84 datum to ECEF can be used as an intermediate step in converting velocities to the north east down coordinate system. Conversions between ECEF and geodetic coordinates are discussed at geographic coordinate conversion. Geocentric coordinates can be used for locating astronomical objects in the Solar System in three dimensions along the Cartesian X, Y, Z axes, they are differentiated from topocentric coordinates, which use the observer's location as the reference point for bearings in altitude and azimuth. However, neither system takes Earth's constant motion into account, which requires the addition of a time component to fix objects. For nearby stars, astronomers use heliocentric coordinates, with the center of the Sun as the origin; the plane of reference can be aligned with the Earth's celestial equator, the ecliptic, or the Milky Way's galactic equator. These 3D celestial coordinate systems add actual distance as the Z axis to the equatorial and galactic coordinate systems used in spherical astronomy.
The distances involved are so great compared to the relative velocities of the stars, that for most purposes, the time component can be neglected. Geodetic system Earth-centered inertial coordinate system International Terrestrial Reference System Orbital state vectors ECEF datum transformation Notes on converting ECEF coordinates to WGS-84 datum Datum Transformations of GPS Positions Application Note Clearer notes on converting ECEF coordinates to WGS-84 datum geodetic datum overview orientation of the coordinate system and additional information GeographicLib includes a utility CartConvert which converts between geodetic and geocentric or local Cartesian coordinates; this provides accurate results for all inputs including points close to the center of the earth. EPSG:4978
International System of Units
The International System of Units is the modern form of the metric system, is the most used system of measurement. It comprises a coherent system of units of measurement built on seven base units, which are the ampere, second, kilogram, mole, a set of twenty prefixes to the unit names and unit symbols that may be used when specifying multiples and fractions of the units; the system specifies names for 22 derived units, such as lumen and watt, for other common physical quantities. The base units are derived from invariant constants of nature, such as the speed of light in vacuum and the triple point of water, which can be observed and measured with great accuracy, one physical artefact; the artefact is the international prototype kilogram, certified in 1889, consisting of a cylinder of platinum-iridium, which nominally has the same mass as one litre of water at the freezing point. Its stability has been a matter of significant concern, culminating in a revision of the definition of the base units in terms of constants of nature, scheduled to be put into effect on 20 May 2019.
Derived units may be defined in terms of other derived units. They are adopted to facilitate measurement of diverse quantities; the SI is intended to be an evolving system. The most recent derived unit, the katal, was defined in 1999; the reliability of the SI depends not only on the precise measurement of standards for the base units in terms of various physical constants of nature, but on precise definition of those constants. The set of underlying constants is modified as more stable constants are found, or may be more measured. For example, in 1983 the metre was redefined as the distance that light propagates in vacuum in a given fraction of a second, thus making the value of the speed of light in terms of the defined units exact; the motivation for the development of the SI was the diversity of units that had sprung up within the centimetre–gram–second systems and the lack of coordination between the various disciplines that used them. The General Conference on Weights and Measures, established by the Metre Convention of 1875, brought together many international organisations to establish the definitions and standards of a new system and standardise the rules for writing and presenting measurements.
The system was published in 1960 as a result of an initiative that began in 1948. It is based on the metre–kilogram–second system of units rather than any variant of the CGS. Since the SI has been adopted by all countries except the United States and Myanmar; the International System of Units consists of a set of base units, derived units, a set of decimal-based multipliers that are used as prefixes. The units, excluding prefixed units, form a coherent system of units, based on a system of quantities in such a way that the equations between the numerical values expressed in coherent units have the same form, including numerical factors, as the corresponding equations between the quantities. For example, 1 N = 1 kg × 1 m/s2 says that one newton is the force required to accelerate a mass of one kilogram at one metre per second squared, as related through the principle of coherence to the equation relating the corresponding quantities: F = m × a. Derived units apply to derived quantities, which may by definition be expressed in terms of base quantities, thus are not independent.
Other useful derived quantities can be specified in terms of the SI base and derived units that have no named units in the SI system, such as acceleration, defined in SI units as m/s2. The SI base units are the building blocks of the system and all the other units are derived from them; when Maxwell first introduced the concept of a coherent system, he identified three quantities that could be used as base units: mass and time. Giorgi identified the need for an electrical base unit, for which the unit of electric current was chosen for SI. Another three base units were added later; the early metric systems defined a unit of weight as a base unit, while the SI defines an analogous unit of mass. In everyday use, these are interchangeable, but in scientific contexts the difference matters. Mass the inertial mass, represents a quantity of matter, it relates the acceleration of a body to the applied force via Newton's law, F = m × a: force equals mass times acceleration. A force of 1 N applied to a mass of 1 kg will accelerate it at 1 m/s2.
This is true whether the object is floating in space or in a gravity field e.g. at the Earth's surface. Weight is the force exerted on a body by a gravitational field, hence its weight depends on the strength of the gravitational field. Weight of a 1 kg mass at the Earth's surface is m × g. Since the acceleration due to gravity is local and varies by location and altitude on the Earth, weight is unsuitable for precision
World Geodetic System
The World Geodetic System is a standard for use in cartography and satellite navigation including GPS. This standard includes the definition of the coordinate system's fundamental and derived constants, the ellipsoidal Earth Gravitational Model, a description of the associated World Magnetic Model, a current list of local datum transformations; the latest revision is WGS 84, established in 1984 and last revised in 2004. Earlier schemes included WGS 72, WGS 66, WGS 60. WGS 84 is the reference coordinate system used by the Global Positioning System; the coordinate origin of WGS 84 is meant to be located at the Earth's center of mass. The WGS 84 meridian of zero longitude is the IERS Reference Meridian, 5.3 arc seconds or 102 metres east of the Greenwich meridian at the latitude of the Royal Observatory. The WGS 84 datum surface is an oblate spheroid with equatorial radius a = 6378137 m at the equator and flattening f = 1/298.257223563. The polar semi-minor axis b equals a × = 6356752.3142 m. WGS 84 uses the Earth Gravitational Model 2008.
This geoid defines the nominal sea level surface by means of a spherical harmonics series of degree 360. The deviations of the EGM96 geoid from the WGS 84 reference ellipsoid range from about −105 m to about +85 m. EGM96 differs from the original WGS 84 geoid, referred to as EGM84. WGS 84 uses the World Magnetic Model 2015v2; the new version of WMM 2015 became necessary due to extraordinarily large and erratic movements of the north magnetic pole. The next regular update will occur in late 2019. Efforts to supplement the various national surveying systems began in the 19th century with F. R. Helmert's famous book Mathematische und Physikalische Theorien der Physikalischen Geodäsie. Austria and Germany founded the Zentralbüro für die Internationale Erdmessung, a series of global ellipsoids of the Earth were derived. A unified geodetic system for the whole world became essential in the 1950s for several reasons: International space science and the beginning of astronautics; the lack of inter-continental geodetic information.
The inability of the large geodetic systems, such as European Datum, North American Datum, Tokyo Datum, to provide a worldwide geo-data basis Need for global maps for navigation and geography. Western Cold War preparedness necessitated a standardised, NATO-wide geospatial reference system, in accordance with the NATO Standardisation AgreementIn the late 1950s, the United States Department of Defense, together with scientists of other institutions and countries, began to develop the needed world system to which geodetic data could be referred and compatibility established between the coordinates of separated sites of interest. Efforts of the U. S. Army and Air Force were combined leading to the DoD World Geodetic System 1960; the term datum as used here refers to a smooth surface somewhat arbitrarily defined as zero elevation, consistent with a set of surveyor's measures of distances between various stations, differences in elevation, all reduced to a grid of latitudes and elevations. Heritage surveying methods found elevation differences from a local horizontal determined by the spirit level, plumb line, or an equivalent device that depends on the local gravity field.
As a result, the elevations in the data are referenced to the geoid, a surface, not found using satellite geodesy. The latter observational method is more suitable for global mapping. Therefore, a motivation, a substantial problem in the WGS and similar work is to patch together data that were not only made separately, for different regions, but to re-reference the elevations to an ellipsoid model rather than to the geoid. In accomplishing WGS 60, a combination of available surface gravity data, astro-geodetic data and results from HIRAN and Canadian SHORAN surveys were used to define a best-fitting ellipsoid and an earth-centered orientation for each of selected datum; the sole contribution of satellite data to the development of WGS 60 was a value for the ellipsoid flattening, obtained from the nodal motion of a satellite. Prior to WGS 60, the U. S. Army and U. S. Air Force had each developed a world system by using different approaches to the gravimetric datum orientation method. To determine their gravimetric orientation parameters, the Air Force used the mean of the differences between the gravimetric and astro-geodetic deflections and geoid heights at selected stations in the areas of the major datums.
The Army performed an adjustment to minimize the difference between astro-geodetic and gravimetric geoids. By matching the relative astro-geodetic geoids of the selected datums with an earth-centered gravimetric geoid, the selected datums were reduced to an earth-centered orientation. Since the Army and Air Force systems agreed remarkably well for the NAD, ED and TD areas, they were consolidated and became WGS 60. Improvements to the global system included the Astrogeoid of Irene Fischer and the astronautic Mercury datum. In January 1966, a World Geodetic System Committee composed of representatives from the United States Army and Air Force was charged with developing an improved WGS, needed to satisfy mapping and geodetic requirements. Additional surface gravity observa